Photometric Imprecision Can Limit BUG Rating Utility

Abstract The IES BUG rating system is useful for classifying outdoor luminaires when considering light trespass, sky-glow, and glare in accordance with the joint IDA-IES Model Lighting Ordinance (MLO). There are, however, limits to what can be measured in the laboratory. IES TM-15-11 requires that a luminaire with an uplight rating of U0 emits zero lumens into the upper hemisphere, while a U1 uplight rating or a G0 glare rating for high viewing angles requires fewer than 10 lumens. Given that the luminaire is emitting thousands of lumens and that the laboratory room surfaces have a diffuse reflectance of at least two percent, it is physically impossible to measure zero lumens, and extremely difficult to measure fewer than 10 lumens. Consequently, a U0 glare rating can only be obtained by physical examination of the luminaire and post-processing of the measured photometric data. Similarly, a U1 uplight rating or a G0 glare rating for high viewing angles is likely the result of data manipulation. The MLO User's Guide should therefore be revised to provide a procedure within the ordinance text whereby an exception is allowed if a visual inspection of the physical luminaire shows that the uplight is negligible.